1. Field of the Invention
The present invention relates generally to an inkjet printer system and, more particularly, to a complementary metal-oxide-semiconductor (CMOS) controlled printhead sense circuit in an inkjet printer.
2. Description of the Related Art
A printhead power-up process in an inkjet printer must be performed to ensure the printhead is not damaged. A part of this process is first checking that a printhead is present in the printer and installed correctly. Previously, heater chips in the printer have used a simple PNP transistor, connected to function as a p-diode, as the printhead sense circuit to detect the presence of the printhead. An example of this prior art p-diode printhead sense circuit is illustrated in FIG. 1.
The p-diode printhead sense circuit was originally implemented to function primarily as an electrostatic discharge device (ESD) between logic pins and a logic power rail (LPWR). Its secondary function was as a printhead sense circuit. The inkjet printer could apply a voltage to the temperature sense resistor pin (TSR+) prior to printhead power (VPH) being applied and a current would be drawn from the temperature sense resistor pin to the logic power rail, the control node for the sense circuit. A voltage similar to the typical temperature sense resistor voltage would develop on the temperature sense resistor pin and the sense circuit would recognize that a printhead was present. Then, when the normal printhead operation occurred, the printhead power would increase which would drive the logic power rail control node on the chip high. This would turn the p-diode off and allow for normal temperature sense resistor reading to begin.
This prior art printhead sense circuit has shortcomings. Primarily, they concern the consequences of leakage of current on the input pin. Leakage of current of sufficient magnitude on the input pin can disturb temperature reading and introduce lack of control of the operation of the circuit. While more recent printhead sense circuitry is somewhat more reliable for keeping current leakage on the temperature sense node to a tolerable minimum, less than a micro-Amp, the value of the logic power rail still can droop during operation. So the circuitry presently in use is still in some jeopardy of affecting the temperature sense resistor circuitry.
Thus, the prior art printhead sense circuit still has two major shortcomings, the first being potentially an unacceptable magnitude of leakage current into the temperature sense pin, and the second being an unstable method of holding the printhead sense circuit off during normal printhead operation. As a consequence, there is still a need for an innovation that will overcome these shortcomings.